Since commit 5f71eac0 the Sphinx tool is required
to build the rST documentation.

This fixes:

 $ ./configure --enable-docs

 ERROR: User requested feature docs
        configure was not able to find it.
        Install texinfo, Perl/perl-podlators and python-sphinx

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190711102710.2263-1-philmd@redhat.com>
[AJB: also add /usr/libexec/python3-sphinx/ to PATH]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>